Worldwide 12 million people are infected with the single-celled parasite Leishmania, with 2 million new cases each year, and 350 million people at risk. Leishmaniasis is the number two killer worldwide after malaria for death caused by parasites. Leishmaniasis cases have been increasing due to development and wars moving people into parasite-infested areas. Spread by sandfly bites, these parasites cause fatal disease of the internal organs as well as non-healing and disfiguring diseases of the skin, mouth, and nose. Drug therapies for leishmaniasis have severe side effects and are losing their curative effect, and no practical vaccine has yet been developed. In addition, over 1,000, and possibly 10,000, cases of leishmaniasis have occurred in the U.S. military stationed in Afghanistan and Iraq. We have been studying Leishmania mexicana infection in mice, attempting to understand why mice do not heal, whereas a related parasite, L. major, causes skin lesions that do heal. Human disease caused by L. mexicana can last for years. It is important to understand how L. mexicana stops the body from killing the parasite, in order to develop therapies to block this parasite mechanism in human infections caused by Leishmania and other pathogens that live inside cells (tuberculosis, malaria, and toxoplasmosis). We found that a white blood cell protein, called IL-10, prevents the body's protective immune response and that antibodies (proteins made to help fight infection) can attach to the surface of Leishmania parasites inducing IL- 10 production. Thus the body's own antibodies are exploited by the parasite to prevent healing. When infected with Leishmania, rabbits and mice develop antibodies to a type of molecule with fat and sugars called glycoinositol phospholipids (GIPLs), but not to proteins (which are the usual targets of antibodies). We will test our hypothesis that GIPLs, abundant surface molecules of Leishmania, are the targets of antibody responses that prevent healing. We will also determine if an unusual type of white blood cell called natural killer T (NKT) cells are important to this immune response that prevents healing. These NKT cells are activated by an unusual protein called CD1, which is involved in immune responses provoked by glycolipids (like GIPLs). The way in which CD1 and NKT cells work in infection is not well understood. Although it has long been known that antibody responses do not cure infections caused by organisms that hide inside cells, it is now becoming clear that these antibodies can prevent healing. We believe that understanding the ways that antibodies can prevent healing of leishmaniasis may help us to determine ways to evaluate new vaccines against Leishmania infection. In particular, vaccines may need to be designed that do not cause certain types of antibodies to be made against GIPLs. In this way we may contribute not only to a basic understanding of the immune system but also, to the production of new vaccines against leishmaniasis and diseases caused by other pathogens that hide inside cells, like toxoplasmosis, tuberculosis and malaria. PUBLIC HEALTH RELEVANCE: The single-celled parasite Leishmania, which is the number two killer worldwide after malaria for death caused by parasites, infects over two million people each year, with 12 million infected at any one time, and 350 million people at risk. Understanding the mechanisms of how the parasite prevents elimination by the host is crucial for developing vaccines and drug therapies. Determining how glycolipids (sugars attached to fat) and CD1, a mammalian protein that helps present them to the immune system, work to suppress an immune response will aid in developing novel treatments for leishmaniasis and potentially other pathogens that hide inside cells.